(1) Field of the Invention
The present invention relates generally to the manufacturing of composite materials, and, more particularly to the manufacture of carbon-carbon composites by densifying carbon-carbon composites with a polymeric matrix.
(2) Description of the Prior Art
Carbon-carbon composites are comprised of a carbonaceous matrix reinforced by carbon fibers. Such composites are used to combine the advantages of fiber-reinforced composites with the refractory properties of structural ceramics. These composites maintain their properties at high temperatures making them suitable for uses such as space vehicle heat shields, rocket nozzles and aircraft brakes. Additional applications have been found for these composites in the medical and industrial areas given the composite's biocompatibility, chemical inertness and thermal conductivity.
A common method of manufacturing carbon/carbon composites is by pyrolyzing a carbon fiber/polymeric composite. One example would be a polymeric composite based on autoclave cured carbon fiber/phenolic prepreg. The prepreg is cut and stacked on a molding tool. The assembly of the prepreg and mold are bagged and placed in a autoclave. A vacuum is pulled on the bag and the temperature is raised in the autoclave at a specified rate. At the time the resin softens, the autoclave pressure is increased to about 100 psi. The autoclave is then held at about 350° F. for about two hours. Next, the autoclave is cooled to room temperature and the cured composite is removed from the tool.
The cured composite is placed in a carbonization furnace and heated slowly at a specified rate in the absence of oxygen to about 1000° C. This process often requires a number of days. The heating step volatilizes all of the organic portion of the phenolic molecule except for the carbon. The phenolic resin looses about 35% of its weight during the carbonization process creating microvoids or porosity in the composite. Significantly, these voids extend to the center of the item being manufactured and must be filled in order to maximize the strength of the final product.
The voids are then filled with a solvent diluted resin by series of vacuum and pressure cycles. The carbon/carbon composite is submerged in the solvent solution of phenolic resin (or other high carbon content material such as furan thermoset resin) and evacuated. While still submerged in the solution, pressure is added to further push the solution into the composite porosity. The impregnated composite is then dried to remove the solvent. Care must be taken to remove all the solvent without losing the resin. After all the solvent is removed, the resin must then be cured or otherwise stabilized in the composite pores.
The resin impregnated carbon/carbon composite is then recarbonized by essentially the same carbonization process. If the process fully performs its purpose, the micro void content is reduced by another increment. The amount of this reduction depends on the solvent/resin ratio and the carbon content of the resin used. The solvent/resin solution typically is about 50% resin by volume and a resin with a high carbon yield looses about 35% of its volume during carbonization. This results in about 18% of void volume that is filled by repeated re-impregnation and recarbonization. This process is called densification.
The purpose of densification is primarily to obtain good mechanical properties. As the density increases, so do the mechanical properties of the resulting composite. Three or more re-impregnations (and carbonizations) are sometimes required to achieve a useful product depending on the end use for the composite.
Although the use of carbon-carbon composites has become widespread, the market for these materials has been limited due to their high cost of production. These costs arise in part from the need for the multiple re-impregnations and carbonizations steps. As discussed above, these steps can take days and are susceptible to high production loss rates from minor imperfections in the process.
Thus, there remains a need for an apparatus for forming a densified carbon-carbon composite which provides for a substantial reduction in the number of carbonization cycles required to reach final density while, at the same time avoids the environmental and safety problems of a solvent-based system.